Automotive heat exchangers, such as radiators, heater cores, evaporators and condensers are predominantly made of aluminum alloys to reduce the weight of the vehicles. These heat exchangers can be the tube and fin type where the fins are corrugated and/slotted at right angles to the direction of air flow.
In the past, mechanical expansion techniques have been used for mass-production of automotive finned-tube heat exchangers. Heat exchangers are now predominantly formed by a brazing operation, wherein the individual components are permanently joined together with a brazing alloy.
Since the early 1980s, one brazing technique known as controlled atmosphere brazing (CAB) has become increasingly popular for use by automotive industry to make brazing aluminum heat exchangers. CAB has been preferred over a previous brazing method, i.e., vacuum furnace brazing, due to improved production yields, lower furnace maintenance requirements, greater braze process robustness, and lower capital cost of the equipment employed.
When manufacturing the heat exchangers using the CAB process, an aluminum brazing filler alloy (e.g., AA 4345 or AA 4043) is often pre-cladded or coated on at least one side of the core aluminum alloy sheet (or brazing sheet). Alternatively, a prebraze arc sprayed Zn coating is applied on the non-clad tubes (e.g., via a wire arc spraying process) to improve their corrosion resistance. The aluminum core alloys of the fins and tubes are typically AA 3003 or various “long life alloys” or modified AA 3003 alloys with additions of small amounts of elements typically selecting from Cu, Mg, Mn, Ti, Zn, Cu, Cr and Zr.
In the CAB process, a fluxing agent is applied to the pre-assembled component surfaces to be jointed. During brazing at approximately 560 to 575° C., the fluxing agent starts to melt and the melted flux reacts, dissolves and displaces the aluminum oxide layer that naturally formed on the aluminum alloy surface and frees up the brazing filler alloy. The brazing filler alloy starts to melt at about 575-590° C. and begins to flow toward the joints to be brazed. During the cooling process, the filler metal solidifies and forms braze joints. The flux present on the surface also solidifies and remains on the surface as flux residue.
Additional functions of the fluxing agent are to prevent reformation of an aluminum oxide layer during brazing, enhance the flow of the brazing filler alloy, and increase base metal wettability. The fluxing agent is typically a mixture of alkaline metal fluoroaluminates with general formula K1-3AlF4-6.xH2O, which is essentially a mixture of K3AlF6, K2AlF5 and KAlF4. Fluoride-based fluxes are preferred over chloride based fluxes for brazing aluminum or aluminum alloys because they are considered to be inert or non-corrosive to aluminum and its alloys, and substantially water insoluble after brazing. When the recommended flux coating weight (3-5 gram per square meter (g/m2) for furnace brazing) is used, the CAB process is said to generate a 1-2 micrometers (μm) thick tightly adherent non-corrosive residue. Hence, it is believed that no removal of the flux residue is necessary after the brazing operation.
Due to the reported non-corrosive nature of the flux, its tolerance to brazing assembly fit-up and flexible control, CAB is one of the lowest cost methods for the joining of aluminum heat exchangers. It is now commonly used by the automotive and other industries for manufacturing of heat exchangers.